This Application claims the benefit of Japanese application no. 10-072797 filed on Mar. 20, 1998, which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a data recording method and a data recording system, and more particularly, relates to a data recording method and a data recording system for extracting and recording one data out of a plurality of time-division multiplexed data.
2. Description of the Related Art
MPEG 2 standard is an international standard (ISO/IEC 13818-1) proposed by the moving picture expert group (MPEG). It is a coding system for compressing digital video and digital audio signals.
The MPEG 2 standard adopts a time-division multiplexing system using packets. For instance, video and audio signals are divided into streams called packets. The packets have an adequate length for multiplexing the video and audio signals. The packets of the video signal and of the audio signal are then transmitted in a time-division manner while being switched appropriately.
The MPEG 2 standard also adopts a multiplexing and separating system which can accommodate multi-programming in order to transmit a plurality of program data. The use of this system allows not only video and audio signals composing one program data to be transmitted in the time-division manner, but also video and audio signals composing a plurality of programs to be transmitted in the time-division manner.
A system called xe2x80x9ctransport stream systemxe2x80x9d is one of such multiplexing and separating systems which can accommodate multi-programming. According to the transport stream system, a packetized elementary stream (PES) packet is divided further into a plurality of packets in order for the system to transmit a plurality of program data in the time-division manner. Each packet has a fixed length which is relatively short, i.e., 188 bytes.
In order to receive and reproduce the program data transmitted in the time-division manner in the transport stream system, it is necessary to synchronize an encoder provided in a transmitter and a decoder provided in a receiver (reproducing unit). In the transport stream system, information for setting and calibrating a clock reference is called a program clock reference (PCR). The PCR is transmitted to synchronize the encoder and the decoder.
It is necessary to control the value of the PCR and the time when the PCR arrives at the decoder accurately in order to synchronize the encoder and the decoder by using the PCR. More specifically, the PCR is data of six bytes and the decoder must set the value of the PCR therein at the time when the final byte of the PCR has arrived. Then, using the PCR arrival time and the value of the PCR, the decoder sets or calibrates a system time clock (STC) which is the basis on which the decoding and reproducing processes within the decoder are performed.
More specifically, the decoder has a phase locked loop (PLL) circuit in which a counter outputting the STC is built in. Every time when the PCR arrives, the PLL circuit calculates a difference between the value of the PCR and the value of the STC, converts its result into a control signal, and performs a feedback control based on this control signal. This creates a STC which coincides completely with the PCR arrival period. Consequently, the encoder provided in the transmitter and the decoder provided in the receiver can be synchronized with a high precision.
A technology for receiving multiplexed data transmitted by using the multi-programming multiplexing and separating system as described above and for recording the received multiplexed data is now being developed. When such technology is established, it becomes possible for a broadcasting station to provide a digital satellite broadcasting service to multiplex and transmit a plurality of movies recorded in digital signals to individual users at home. Also, it becomes possible for individual users to record the movies transmitted from the broadcasting station in the digital signals as received.
Two methods have being proposed for recording the multiplexed data transmitted by using the multi-programming multiplexing and separating system.
A first method is to record all of the transmitted multiplexed data. This method, however, has problems in that the amount of data becomes too large because all of the multiplexed data is recorded including unnecessary program data contained in the multiplexed plurality of program data.
A second method is to extract and record one program data out of the transmitted multiplexed data. This method allows recording of only the necessary and desired program data. Thus, the amount of recorded data is reduced compared to the first method. However, it is difficult to achieve the synchronization by means of the PCR in reproducing the recorded program data.
In other words, when the program data once recorded is to be reproduced, it is necessary to set or calibrate the STC of the reproducing unit based on the reproducing time of the PCR and the value of the PCR in the same manner as the case of receiving and reproducing the multiplexed data transmitted from the transmitter. However, when extracting and recording one program data out of the time-division multiplex signals, the relationship between the reproducing time of the PCR and the value of the PCR is destroyed. As a result, the STC of the reproducing unit cannot be set or be calibrated based on the PCR, and the program data cannot be reproduced.
Accordingly, the present invention is directed to a data recording method and a data recording system that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a data recording method and a data recording system which allow one specific data to be extracted and recorded out of multiplexed data in which a plurality of data are time-divided. Also, the present invention allows the recorded data to be reproduced with a high precision.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described, a data recording method of the present invention performs the function of receiving multiplexed data arranged such that (i) a plurality of program data are time-division multiplexed by packets, (ii) a plurality of time preset values used in synchronizing one program data with a decoder in decoding at least one program data out of the multiplexed plurality of program data by the decoder are provided in advance at predetermined intervals, and (iii) one program data and the decoder are synchronized based on the value of each time preset value received by the decoder every time when it is received, and extracting the respective packets composing at least one program data and recording the extracted respective packets with a predetermined bit rate. The data recording method of the present invention includes the steps of receiving the time preset values disposed in the multiplexed data; generating a new time preset value based on the time preset values received in the receiving step and on the bit rate; and replacing the time preset value disposed in advance with the new time preset value generated in the generating step in recording the packet in which the time preset value has been disposed in advance out of the respective packets composing one program data.
The plurality of program data are time-division multiplexed by packets in the multiplexed data. The program data include sound data, picture data, image data, character data, control data and signal processing data or data in which all or part of the above are combined.
The time preset values are disposed in advance in the multiplexed data at predetermined intervals and are actually recorded in the packets. For instance, the time preset values are generated by an encoder or the like on the transmitting side and are inserted to the multiplexed data at predetermined intervals. It is noted that there exist packets in which the time preset value is recorded and packets in which no time preset value is recorded in the multiplexed data because the intervals at which the respective time preset values are disposed do not necessarily coincide with the length of each packet.
The time preset values are used to synchronize a specific program data and the decoder in decoding and reproducing the specific program data within the multiplexed data by the decoder. For instance, the decoder decodes and reproduces the specific program data based on a synchronizing signal (es, an internal clock signal) generated within the decoder. Then, the decoder decodes and reproduces the program data while setting or calibrating the synchronizing signal based on the time of receiving the time preset values and based on the values of the time preset values. As a result, the synchronization between the program data and the decoder is established.
In the data recording method of the present invention, at least one program data is extracted out of the multiplexed data in which a plurality of program data are multiplexed. The extracted program data is then recorded with the predetermined bit rate. In this case, it is necessary to generate a new time preset value for establishing synchronization in decoding and reproducing the extracted and recorded program data. This is because the time preset values disposed in advance in the multiplexed data cannot be used for this purpose given that the data array of the program data extracted and recorded out of the multiplexed data is different from the data array of the multiplexed data. This is also because the bit rate at the time of recording is a bit rate of a recording unit itself used in recording the program data.
Therefore, according to the data recording method of the present invention, a new time preset value is set by executing a series of processes from the receiving step through the replacing step described above concurrently with the process of extracting and recording the program data.
In other words, the respective time preset values disposed in the multiplexed data are received at first in the receiving step. Next, a new time preset value is generated in the generating step based on the respective time preset values received in the receiving step and on the above-mentioned bit rate. That is, when the time preset values disposed in advance in the multiplexed data are received, the time when they are received or the values corresponding to the received time is calculated based on the received time preset value and the bit rate. The result is set as the new time preset value in the generating step.
Then, the pre-disposed time preset value is replaced with the new time preset value generated in the generating step in recording the packet with the pre-disposed time preset value among the respective packets composing one program data. Thereby, the pre-disposed time preset value in each packet composing one program data extracted out of the multiplexed data is replaced one after another with the new time preset value calculated based on the bit rate at the time of recording.
Accordingly, when recording of one program data has ended and when the recorded one program data is to be decoded and reproduced with the same bit rate at the time of recording, the synchronization between the one program data and the decoder (reproducing unit) may be established with a high precision. Consequently, the recorded one program data may be decoded and reproduced with a high precision by using the new time preset values disposed in the recorded program data.
It also allows extraction and recording of only one program data out of the multiplexed data in which the plurality of program data are multiplexed. This reduces the amount of data to be recorded.
According to a first aspect of the present invention, the multiplexed data includes a plurality of program data which are time-division multiplexed by packets of a fixed length; and the new time preset value T is calculated in the generating step by using a number of packets N composing one program data extracted from the multiplexed data during the time from when the previous time preset value was received till when the time preset value of this time is received in the receiving step. The new time preset value T is also related to the length L of those packets, time T0 when the previous time preset value was received, and the bit rate R, as follows:
T=(Nxc3x97L/R)+T0xe2x80x83xe2x80x83(1)
In other words, in the generating step, the data length of the data recorded during the time from when the previous time preset value has been received till when the time preset value of this time is received is calculated by (number of packets N)xc3x97(packet length L). Then, the time from when the previous time preset value has been received till when the time preset value of this time is received is calculated by dividing the data length by the bit rate R at the time of recording. Further, the time when the time preset value of this time is received is calculated by adding the time T0 when the previous time preset value has been received to this value. Then, the time when the time preset value of this time is received is set as the new time preset value T.
Using the above calculation, the time preset value used in establishing the synchronization in decoding and reproducing the extracted and recorded program data may be readily generated. It is noted that the time preset value of the previous time does not mean only the time preset value just before the time preset value of this time. The time preset value of the previous time means the time preset values received during the time since when the one program data has started to be recorded till when the time preset value of this time is received. In other words, the previous time preset value can be a time preset value received for the first time since when the one program data has started to be recorded.
Also, the present invention can be used with respect to the multiplexed data including a plurality of program data which have been time-division multiplexed by the transport stream system in the MPEG, and each time preset value is PCR in the MPEG.
More specifically, the packets composing one program data are extracted out of the multiplexed data in which a plurality of program data are time-division multiplexed by the transport stream system in the MPEG. The extracted packets are then recorded with the predetermined bit rate. At this time, the PCR disposed in advance is replaced with the new PCR generated based on the bit rate at the time of recording the packet having the pre-disposed PCR.
According to the present invention, the bit rate can be set by dividing the clock frequency used in counting the value of the respective time preset values arrayed in advance in the multiplexed data. As a result, the bit rate may be set by dividing the clock signal for generating the clock used in counting the values of the respective time preset values arrayed in advance in the multiplexed data. Accordingly, the bit rate may be set with a simple arrangement without providing an additional oscillator. Only a frequency divider is required. Further, the new time preset value generated in the generating step may be set at a value equal to 1/n of the time preset value disposed in advance in the multiplexed data, so that the process for establishing the synchronization in reproducing the program data extracted and recorded out of the multiplexed data may be simplified.
In another aspect, the present invention provides a data recording system which utilizes the data recording method described above.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.